1. Field of the Invention
This invention relates to an optical device. More particularly it relates to devices for performing one or more of the functions of beamsplitting, recombination or mixing, and to devices employing such functions.
2. Discussion of Prior Art
Optical devices which accept input light beams and perform beamsplitting, recombination or mixing functions are known in the prior art. Such devices include beamsplitters and prisms, which divide one inputbeam into two or mix two input beams to produce two mixed output beams. These are bulk components suitable for use in free space optics; they suffer from stability, bulk and cost limitations, and are not easily applicable to integrated optics.
Optical devices for beamsplitting etc may also be constructed from optical fibres. These may for instance be fused fibre couplers, or directional couplers. Like bulk components, such fibre components suffer from limitations in their applicability to integrated optics. In addition efficiency of injection of light beams into the fibres is generally low and the efficiency of splitting or recombination is generally low. Typical splitting losses in fused fibre couplers are 75%.
A variety of devices for beamsplitting and related functions is also known in planar waveguide technology. These are more readily applicable to integrated optics than bulk and fibre optic devices. They are mostly based on Y-junctions, in which efficiency of splitting or recombination is low. Injection of light beams into these devices is normally inefficient, as is the case for fibre optic components.
In "Passive Paths for Networks", Physics World, September 1991, pp 50-54 T. Ikegami and M. Kawachi briefly discuss the forms of devices described above, in isolation from considerations of input to them.
Due to the increasing importance of optical networks and integrated circuits, much work has been undertaken to improve the efficiency of splitting and recombination in fibre or planar waveguide Y-junctions and associated devices. Some workers, such as Z. Weissman, A. Hardy and E. Marom in IEEE Journal of Quantum Electronics, Vol 25, No 6 (1989) pp 1200-1208, have considered basic modes propagating within the devices. However, they found that splitting efficiency was still heavily dependent on the angle within the Y-junction, and was reduced to 20% at 7.degree.. Other workers have developed active Y-junctions; one such is described by H. Sasaki and I. Anderson in "Theoretical and Experimental Studies on Active Y-Junctions in Optical Waveguides", IEEE Journal of Quantum Electronics Vol QE-14, No. 11 (1978) pp 883-892. These authors found that efficiency fell off rapidly with increasing split angle between Y-junction outputs, and was only 20% at a split half angle of 3.degree..
Asymmetric Y-junctions are also known. One such device, using total internal reflection to achieve higher efficiency at the split, is described by K. Shirafuji and S. Kurazono in "Transmission Characteristics of Optical Asymmetric Y-Junction with Gap Region", Journal of Lightware Technology Vol 9, No. 4 (1991) pp 426-429. All three references discuss the operation of the respective devices and do not consider input to them.
Other forms of optical devices for beamsplitting etc. are also known in planar waveguide technology, such as combinations of Y-junctions forming more complex devices and star couplers. There is, however, an unfulfilled need for optical devices which accept input beams and perform efficient beamsplitting, recombination or mixing. In particular there is a need for such devices which are readily applicable to integrated optical circuits.